Compression steel ?

[crap]

Hi you guys,
I'm curious that there is a situation that we have the compression steel in 2 ways slab or not? As I known we try to design the slab as singly reinforcement, right? Please share you idea
Thanks
CCCP

 

[ishvaaag]

This is just a convenience for both design and construction. The 1 way idealizations are just such but with the care exacted by the codes experience proves are perfectly valid solutions to construction design problems.

Furthermore, setting the beams and joists in their 1 way layout, and being the structure forced to use -to counteract the load- the minimum energy of deformation, i.e., to assume the deformation in the more efficient way, soon it finds that the more efficient way is precisely to act against the more rigid subsystems present, i.e., to follow the 1 way paths we have endowed it with.

In other words, the 2 way effects become lesser than would be in the 2 way slab arrangement, or maybe we should better say become channelized through the laid resisting systems, joists and beams.

 

[JAE]

I have not seen any situations where compression steel was used in a 2-way slab system. Usually, in a 2-way system, if you need compression steel, you probably have a too thin slab that would give you deflection problems.

Also, with compression steel, it would be very difficult and expensive to provide tie confinement.

 

[rapt]

Ishvaaag,

That was not the question. To comment on your reply first: Using a one way system for a 2way slab as you suggest is not the best thing to do. It may work in ultimate strength but serviceability problems will be encountered in both crack control and deflections. If the elastic moments want to be 2way then that is how you should design it, otherwise large amounts of redistribution will be required to change from a 2way load path to a 1way load path. This redistribution is accompanied by increased cracking, rotation, deflection and ductility problems.
The only time this method is used is for lightly stressed PT slabs to USA practice and in those cases, the stresses in the concrete need to be limited to ensure no cracking occurs at service. It should never be used for RC slabs.

CRAP
JAE is correct in that compression face reinforcement is ineffective in slabs. You will notice I said "face" because in a thin slab, this reinforcement will notmally be in the tension zone and so will not actually be in compression. I have seen designers use it but it is really no use in these cases.

ACI and some other codes use a long term deflection multiplier to allow for shrinkage and creep deflections which indicates that compression face reinforcement has a significant effect on long term deflections. This formula is not correct for slabs and lightly loaded T-beams.
The effect of this compression face reinforcement will only be significantly helpful for more heavily loaded deeper beams where the reinforcement will be significantly in compression.

 

[Bannu]

crap

I agree with rapt.

When the thickness of the slab is high enough say more than 8", we may some times go for compression reinforcement to bear differential stresses due to temparature variations.

In other cases compression reinforcement in slabs may not be economical and practical.

 

[zliazmei]

I agree with rapt.

In rectangular section with top and bottom reinforcement.
The compresion one should not be considered in calculations if the depth of compresion zone is less than 2 times the deth to the comperion reinforcement.
Means for thin slab it does not work.
In basemats with the high dept you will see that X>2d' so it will work.

In all other cases the compresion reinforcement is plased to ressist temperature and other long therm effects.

Regards

Zlia Zmei

 

[v2]

Increase the slab thickness or reduce the span. but do not use compression steel in 2-way slab. if you find yourself in need for a compression steel that should tell you that you have a flaw in your floor framing. you need to reconsider and redesign your slab configuration.